As is known, the use of electromechanical micro-structures of semiconductor material, the manufacture of which utilizes microelectronics techniques, has recently been proposed for producing accelerometers and gyroscopes. These silicon micro-processing techniques make it possible to manufacture different types of angular velocity and acceleration sensors. In particular, at the present time prototypes operating according to the piezoelectric, piezoresistive, capacitive, threshold, resonant and tunnel effect principle of operation have been proposed.
Historically, integrated micro-structures have been manufactured by preferably using the bulk micromachining technique in which a wafer of single-crystal silicon is processed on both faces. This technique is, however, incompatible with the process phases for forming components of the circuit which processes the signal picked up by the sensitive element.
It has therefore been proposed to use the technique of surface micromachining in which the sensitive element is made of polycrystal silicon and suspended structures are formed by depositing and successively removing sacrificial layers. This technique is compatible with the current integrated circuit manufacturing processes and is therefore preferred at present. The integrated micro-structures formed with this technique are, however, relatively insensitive to acceleration and angular velocity. In fact, having a mass of the order of a few tenths of a microgram, they suffer the effects of thermodynamic noise caused by the Brownian movement of the particles of the fluid in which they are immersed (see, for example, the article by T. B. Gabrielson entitled "Mechanical-Thermal Noise in Micromachined Acoustic and Vibration Sensors", IEEE Transactions on Electron Devices, vol. 40, No. 5, May 1993). The upper limit to the mass obtainable with these structures is imposed by genuinely technological reasons; the deposition of very thick films involves extremely long wafer processing times and renders the surface of the wafer unsuitable for the successive operations such as the lapping of the wafers.